Conventional lens meters of ocular type operate to measure a refractive power of an inspected lens by moving a measurement target such that it is focused on a scale plate while observing a measurement target image projected onto the inspected lens through an observation optical system thereof.
In this type of lens meter, optical axes of a measurement optical system and an observation optical system are made coincident with each other. The optical axis of the optical system is inclined relative to a plane on which the lens meter is installed. This optical axis is rotatable in a range from 0 to 90 degrees.
The conventional lens meter is provided at its different positions with two knobs, i.e., a power adjustment knob for moving the measurement target in the optical axis direction and an axis adjustment knob for rotating the measurement target. Thus, a sphere power, a cylinder power, a cylinder axis or the like can be measured by operating the above two knobs of the lens meter.
Other conventional lens meters include a so-called auto-off timer circuit. At the same time when a power switch of the lens meter is turned on, the auto-off timer circuit is automatically actuated and then turns off the power switch of the lens meter after a set time was passed.
The above lens meter needs a wide plane on which it is installed so that it can be made stable at any angle from 0 to 90 degrees at which the optical axis of the optical system is inclined. However, since a workshop of an optician who needs such a lens meter is generally small, the conventional lens meter that requires a wide installment area is not desirable.
Further, if the optical axis of the optical system is rotated nearly 90 degrees, there is then the disadvantage such that the inspector cannot see a marking device and a lens table substantially.
Furthermore, according to the above conventional lens meters, the inspector must operate the knobs while observing the measurement target image through the ocular tube. Accordingly, if the inspector measures an cylinder lens which forces the inspector to operate both the power adjustment knob and axis adjustment knob is measured, there is then the disadvantage such that the inspector has to make his eye off from the ocular tube each time the inspector confirms the position of the knob or that the inspector has to feel the knobs while observing the measurement target image through the ocular tube.
In addition, according to the above conventional lens meter with the auto-off timer circuit in which the auto-off timer circuit is actuated at the same time when the lens meter starts to operate, the power switch is turned off after a predetermined time was passed since the lens meter has been actuated. Accordingly, when several lenses are continuously measured or lenses such as a progressive addition lens that needs a long measurement time are measured, there is then the risk that the power switch of such lens meter is suddenly turned off during the measurement. To remove such risk as much as possible, a time set by the auto-off timer circuit must be extended, which unavoidably loses the auto-off function substantially.